When a new connection is to be set up by a switch in a telecommunications network, the switch has to assign certain resources, i.e., buffer to hold the incoming cells and bandwidth to transmit cells, according to the specified Quality of Service (QoS) requirement and the connection parameters. QoS can be specified in terms of the amount of delay and/or cell loss ratio. The connection parameters include average sending rate, peak sending rate, and maximum back-to-back cell number (maximum burst size). Initially, the resource assignment is static and remains unchanged during the lifetime of the connection. Therefore, if more resources become available, a static resource connection assignment cannot make use of the additional available resources to achieve better performance.
In some conventional switches, dynamic adjustment of the buffer (memory) allocation for connections based on the overall queue occupancy is performed. When most connections are active, and the total buffer occupancy is high, the per connection buffer allocation is low. When the total buffer occupancy is low, the per connection buffer assignment is high, permitting active connections using more buffer to achieve lower cell loss rate.
The same requirement is true for bandwidth assignment. There are three conventional scheduling methods, namely strict priority, rate scheduling, and weighted round robin. Strict priority scheduling is the simplest method. Strict priority scheduling always gives connections in one class higher priority than connections in another class. For example, whenever any connection in class A has cells to transmit, cells of connections in class B are not sent. Strict priority is risky since there is no control over how much of the class B connections will be made. Therefore, it is difficult to support more than two classes of service. Furthermore, service guarantee can only be made for the highest priority class, and not to the others. Strict priority also cannot be used for scheduling allocation within one class.
The rate scheduling method uses a timed ring to make the cell-to-cell spacing of a connection equal to a predefined value, which is the inverse of the connection's assigned bandwidth. When the assigned bandwidth is static, the cells associated with the connection cannot be sent faster than the assigned bandwidth.
Weighted round robin (WRR) is another conventional scheduling method. It uses the weight assigned to a connection to make the scheduling decision. The actual bandwidth allocated to a connection is proportional to its own weight and inversely proportional to the total weight of all active connections. Therefore, when other connections are not using their bandwidths, i.e. they have no cells to transmit, a connection can automatically get more bandwidth. WRR scheduling can provide a connection with the assigned bandwidth and make a full use of the available bandwidth as long as the switch bounds the total weight of active connections by rejecting some connections. For example, if there are a total of ten connections and each with the weight of 2. If all connections are active, a connection is allocated 1/10 link bandwidth. If only five connections are active, then an active connection is now allocated 1/5 link bandwidth. However, this method uses static weight assignment, which only occurs at connection setup. Given a bounded total weight, the weight assigned to a connection with a delay guarantee has to be large enough to make sure that the connection obtains sufficient bandwidth without violating the QoS service contract. If the weight assignment is conservative and based on the largest possible sending rate, the switch can obey the service contract but only support a very limited number of connections that require strict QoS guarantee. On the other hand, if the weight is assigned based on the average sending rate, it is possible that the cells experience large delay and are useless to the receiver. Internet service providers (ISPs) prefer to assign minimum weight to a connection in order to support more connections for higher profit and still adhere to the service contract. WRR with static weight assignment cannot fulfill these goals.